E. coli aspartate transcarbamoylase (ATCase) is a classical allosteric protein whose function is thought to be controlled primarily by a quaternary structure transition between two states. The E50A mutant is one of a few ATCase mutants for which a third state different from T or R state in the two state model has been suggested by biochemical studies. We conducted time-resolved solution scattering studies on kinetics of the quaternary structure transition of the mutant immediately after mixing with a substrate solution. The structure change of this mutant was found to take place on the order of seconds, a rate much slower than that of wild-type ATCase thus eliminating the use of the cryoenzymology technique. The substrate-bound form of E50A appeared to be in a quaternary structure intermediary between T and R states, and effects of an allosteric effector ATP and bisubstrate analog PALA on the kinetics were studied.